A visit to the collection of royal manuscripts at the British Library is an experience of rich, almost suffocating beauty. The books, deeply coloured and elaborated with gold leaf, stand as a testament to the skill of their artists – or "my genius", as one illuminator put it. They are also a window on to their patrons and recipients, offering a glimpse of the beliefs and aspirations of medieval English monarchs. Here, too, the politics of late 15th-century England are laid bare.

In a largely illiterate age, visual culture was loaded with political symbolism. Coats of arms and badges – emblems such as Henry VI's antelope and Richard III's boar – were easily identifiable, a stamp of authority. Pinned to jackets, embroidered on banners, carved and moulded into buildings, and painted on shields and manuscripts, they quickly became associated with a particular family through simple repetition.

Looking at the manuscripts that form the core of the collection, the massive, opulent products of 15th-century Flemish ateliers associated with the court of the Yorkist King Edward IV, it is strikingly apparent how they were appropriated and adapted by the most flagrant usurper of them all, the half-blooded Lancastrian exile who, with a meagre claim to the throne, won the battle of Bosworth and founded England's most notorious dynasty, the Tudors: Henry VII.

In the manuscripts that Henry "inherited", existing coats of arms and owners' names are scraped away and overpainted with his own emblems, the red dragon and white greyhound. In one naively drawn example, occupying a full page, the crown of England is depicted nestling in a hawthorn bush, where, according to Tudor myth, it was found after Bosworth; below it an acclamation reads "Vive le noble roy Henry". Elsewhere, the addition of a kneeling Henry VII, crowned and in his robes of estate, intrudes on the eternal calm of the Holy Trinity; at the foot of the same page is another dragon-and-greyhound borne coat of arms, while in the right-hand margin is a baldly drawn red rose; a clutch of smaller red roses nestles in the top left-hand corner. Henry's red rose, intertwined with the Yorkist white rose, has been painted everywhere. In the words of one eminent scholar, these manuscripts have been thoroughly "Tudorised".

If all this seems the act of a nouveau royal family desperate to create an impression, this is precisely what it was. The "Lancastrian" red rose was an emblem that barely existed before Henry VII. Lancastrian kings used the rose sporadically, but when they did it was often gold rather than red; Henry VI, the king who presided over the country's descent into civil war, preferred his badge of the antelope. Contemporaries certainly did not refer to the traumatic civil conflict of the 15th century as the "wars of the roses". For the best part of a quarter-century, from 1461 to 1485, there was only one royal rose, and it was white: the badge of Edward IV. Edward's rose was ubiquitous, blooming on royal seals, on coins and in the bulky manuscripts that he began to acquire consistently from the 1470s onwards. But Edward's death, and the usurpation of his teenage sons by their uncle Richard III, presented an opportunity to the man who would become Henry VII: the exiled Henry, Earl of Richmond, a focus for disaffected Yorkists and Lancastrians alike.

In the year before his invasion of England, Henry's image underwent a thorough makeover. He and his advisers realised that his claim, flimsy as it was, had to be made with the greatest conviction. His letters into England seeking military backing bore the regal monogram "H", while – a play to his Welsh ancestry – he adopted the red dragon of the mythical British king Cadwalladr. And, searching for an appropriate royal emblem, he dusted off the red rose.

This was a masterstroke. Its Lancastrian resonances allowed him to present his reign in visual terms. His accession and marriage to Elizabeth of York, daughter of Edward IV, would unite a traumatised country. The red rose merged with the white to form a new emblem, the Tudor rose, embodied in their offspring. As the Tudor manuscript library grew, the red and red-and-white roses became rooted, established and confident, from the Imagination of vraie noblesse, a book on the definition of true nobility, to an astrological treatise predicting the longevity of Henry's queen, Elizabeth. (Its author, William Parron, fled the country when she died weeks after he had presented the book to Henry.)

History, of course, is written by the victors, and in the aftermath of Henry's accession, the myths were already being created. As one chronicler put it: "In the year 1485 on the 22nd day of August the tusks of the Boar were blunted, and the red rose, the avenger of the white, shines on us." Future generations would turn the story into one of the most enduring narratives of English history: that this had been a war of two "roses", Lancaster and York, which Lancaster – and Tudor – had won in 1485, at the battle of Bosworth field. It had been nothing of the kind.

• Thomas Penn's The Winter Kingis out in paperback from Penguin. Royal Manuscripts: The Genius of Illumination at the British Library ends on 13 March.